No, if the forces have the same sign and are operating in the same direction they add.

You have studied vectors a bit, right? Forces are vectors and the best way to add them is to break the forces down into horizontal and vertical components? See my picture, the red and blue arrows are the components. Equilateral triangles have what angles? Use sin, cos functions to compute the components then add, paying attn to sign! There are two balls each effecting the third, so my picture isn't complete.